Method of injection molding a foamable thermoplastic material

ABSTRACT

Plastic molding machine, primarily adapted for generating and molding a closed-cell foam thermoplastic material. The foam is generated by introducing a gas inert to the thermoplastic material into a granular mixture of same so that the gas is thoroughly intermixed among the granular particles. The gas is then held under pressure while the granular particles are reduced to a flowable material, said pressure being sufficient to assure that the gas will not be expelled therefrom during such process. In one preferred embodiment, gas is mixed into the granular thermoplastic in a hopper and held therein under pressure as said granular material is fed into a screw and plasticated thereby. 
     The plastic material is then conducted at a relatively low pressure, such as that generated by the extrusion machine used for plasticating the material, through a branched system past a check valve in each branch of said system, to an injection assembly. As each injection assembly receives a predetermined amount of plastic material, means are energized preventing more plastic material from entering thereinto but continued operation of the pressure generating device will continue filling other injection assemblies for as long as any thereof are capable of receiving such material. When all injection assemblies are filled, same are actuated simultaneously to drive the plastic material therefrom into the mold cavity. Injection assemblies may supply mold cavities in any pattern, and a single mold cavity may be associated with a given injection assembly or two or more injection assemblies may supply any one given mold cavity. 
     Local zones, preferably subjected to rapid cooling, are provided with undercut notches connected to the sprue, runners or cavity and filled thereby with plastic. Same is cooled ahead of the cooling of the molded part and provides means for holding the mold closed. This permits removal of the mold from the machine prior to cooling of the molded part and opening of the mold which in turn permits more effective use of multiple molds with a given machine.

This is a division of application Ser. No. 233,213, filed Mar. 9, 1972and now U.S. Pat. No. 3,806,291.

FIELD OF THE INVENTION

The invention relates to a plastic molding method and apparatus and itrelates particularly to a type thereof particularly adapted forgenerating a closed cell plastic foam and for molding same. Said moldingis particularly intended for, but not confined to, the molding of verylarge products having dimensions of the order of several feet in atleast two dimensional directions.

BACKGROUND OF THE INVENTION

While the generation of plastic foam and the molding thereof has beencarried out through a variety of techniques and a variety of types ofapparatus for a number of years, certain limitations have existed inprevious known methods and apparatus which have prevented this type ofmaterial from attaining its full potential scope of use. There is a widerange of variables which are applicable to greater or lesser degreesaccording to the particular job to be molded and certain ones arediscussed below to illustrate at least some of the difficulties whichhave been encountered in the prior practice.

For example, in the previous practice, it has been common to place a gasgenerating medium in the thermoplastic material itself which medium willthen produce gas when heated to a predetermined level. The mixture ofthe thermoplastic material and such gas generating material is thenheated, usually by a combination of agitating and of external heatingsources, but conceivably by either acting alone. The foam material isformed thereby and is formed within the plastic heating apparatus. Saidmaterial is then held under sufficient pressure to minimize theexpansion of the gas and the foam, still under such pressure, is thenconducted to the mold, either directly or after preliminary gathering inan accumulator. This procedure is undesirable in that at first the gasgenerating material is often expensive, second it may or may not beuniformly mixed throughout the granular material, and often does notgenerate gas bubbles uniformly within the plastic material, and third,since the gas generating material is responsive at least to some degreeto temperature in controlling the amount of gas generated, and since theheating applied within the plasticizing device does not occur uniformly,the distribution of gas so generated throughout the plastic material isnot uniform and hence further resulting in a foamed plastic productwhich is not of uniform or reliable quality.

Further, in the distribution of such plastic material from the foamgenerator to the mold cavity, present methods are reasonablysatisfactory so long as only a single cavity mold is involved. However,where multiple cavities are involved or the cavities are separated fromeach other by substantial distances, then the pressure drop within theplastic material as same flows through the distribution system from oneof the entrances of the mold cavity to the entrance of the next moldcavity, is often so great that the degree of filling of each moldcavity, or at least the density of material filled thereinto, isvariable and it is extremely difficult to produce satisfactory products.This is particularly difficult where the plastic to be handled is amaterial of high viscosity, such as ABS materials.

Accordingly, the objects of the invention include:

1. To provide a method and apparatus for foaming a thermoplasticmaterial which will not be heat-dependent and will produce a foam ofhighly uniform density and with uniform and evenly distributed internalopenings.

2. To provide a method and apparatus, as aforesaid, in which the gaseousmaterial is introduced into and evenly distributed throughout thethermoplastic material while in a granular state and before theplasticating thereof.

3. To provide a method and apparatus wherein the gaseous material isdistributed uniformly throughout the granular material so that it isthen heated to effect plastication thereof while the gaseous material isheld under sufficient pressure as to maintain itself uniformlydistributed throughout the plastic material and thereby effect theformation of a desirable foam.

4. To provide a method and apparatus for producing a foamed material ofuniform and accurately controllable density.

5. To provide apparatus for carrying out the foregoing claimedprocedures accurately and reliably, which apparatus is of sufficientsimplicity as not to be unreasonably expensive in the manufacturingprocedure nor unreasonably complex to maintain in satisfactory workingcondition.

6. To provide apparatus as aforesaid wherein any kind of thermoplasticmaterial may be mixed with any desired type of gas which is chemicallyinert and otherwise compatible therewith.

7. To provide apparatus as aforesaid wherein the distribution of thefoamed plastic material is carried out at a relatively low pressurewhereby to minimize the pressure losses occurring during suchdistribution operation.

8. To provide apparatus as aforesaid in which the plastic material sodistributed may be received in a plurality of suitable injectionassemblies and wherein completion of the filling of one thereof will notadversely affect the filling of others thereof.

9. To provide apparatus as aforesaid wherein the quantity to beintroduced into each injection assembly is independently adjustable anda change in the adjustment of one injection assembly will not affect theamount of plastic material introduced into another injection assembly.

10. To provide apparatus as aforesaid in which the longer portion of thedistribution system between the foam generating device and the moldcavity is traversed by the plastic material at a relatively lowpressure.

11. To provide apparatus as aforesaid in which the portion of thedistribution system traversed by the plastic material under a relativelyhigh injection, mold filling, pressure is relatively short and of equallength.

12. To provide apparatus as aforesaid in which the molds are arranged incooperation with the plastic being molded therein to provide sufficientforce opposing separation of the mold halves to permit removal of a moldfrom the molding machine prior to such cooling of the part as to permitopening of the mold.

Other objects and purposes of the invention will be apparent to personsacquainted with apparatus of this general type upon reading thefollowing disclosures and inspecting the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a somewhat schematic, partial side view, partially brokenaway, of a molding machine capable of performing the process concepts ofthe invention and embodying the apparatus concepts of the invention.

FIG. 2 is a section taken on the line II--II of FIG. 1.

FIG. 3 is a section taken on the line III--III of FIG. 1.

FIG. 4 is a section taken on either of the lines IV--IV of FIG. 1 orIV--IV of FIG. 3.

FIG. 5 is a detail of the portion within the broken line box "V" of FIG.3.

FIG. 6 is a section taken on the line VI--VI of FIG. 3.

FIG. 7 is a central, sectional detail of the sprue opening in the upperand lower halves, showing same in central section and showing lockingmeans for utilizing the plastic material to lock the mold halvestogether during cooling.

FIG. 8 is a central sectional view of one preferred form of hopper, saidsection being the same as that in FIG. 2.

DETAILED DESCRIPTION

Inasmuch as the process concepts of the invention will be bestunderstood when described in connection with specific apparatus by whichsame may be carried out, the description will proceed directly to aspecific embodiment of the apparatus aspects of the invention. It willbe readily recognized however, that, in spite of the advantagesattributable to the apparatus of the invention which will becomeapparent as the description progreses, the process steps are capable ofbeing practiced by a variety of other specific pieces of equipment.Hence, while the equipment herein illustrated and described will beutilized to illustrate the process of the invention, same is not limitedthereto but may be practiced by other types of equipment.

Turning now to the drawings and with reference first to FIG. 1, there isshown a base 1 supporting a bottom frame plate 2 by suitable standardsof which one appears at 3. Said bottom plate 2 supports a stationaryplaten 4 by a series of rods of which one appears at 5, said rods beingaffixed to the upper and lower frame plates 4 and 3 respectively in aconventional manner such as by having elongated threaded portions 6 ofreduced diameter which extend through said plates and are affixed by anyconvenient means such as nuts 7 and 8. A movable platen 9 is supportedby the plunger 11 of a clamp cylinder 12 and is caused by said cylinderto move vertically as desired in a conventional manner. The stationaryplaten 4 supports the upper, here the fixed, mold half 15. The lowermold half 13 is supported on the movable platen 9 to move upwardlytoward and downwardly away from the stationary platen 4 in a manneralready well known. The clamp cylinder 12 can act in any conventionalmanner in response to a suitable pressure fluid, usually hydraulicfluid, to effect the upward and downward movement.

Thus far, the molding machine as described is conventional and furtherdetailing thereof is unnecessary.

Turning now to the aspects of the apparatus embodying the invention,attention is next directed to FIG. 2 for the means 10 by which the foamis generated and placed under a low but adequate pressure for effectivedistribution of same. Referring to FIG. 2 there is shown a generallycylindrical barrel 16 having an internal cylindrical opening 17extending therethrough. Said opening 17 is occupied by a plasticatingand extrusion screw 18 which will be described in more detailhereinafter. One end of said screw, here the end adjacent the dischargeportion thereof is provided with a drive gear 19 which is then driven inany convenient manner, such as by hydraulic motor 21, which in turn isdriven by any convenient source of hydraulic pressure fluid, all ofwhich is already well known and needs no detailing. The same end of saidscrew is preferably received into and retained by a suitable thrustbearing 22 which bearing may be of any conventional nature and needs nofurther detailing. In this case however, it will be noted that thethrust bearing is of double construction so as to prevent axial movementof the screw in both axial directions.

The other end of said opening 17 is preferably closed by a cap 23through which extends an opening 24, said opening 24 being connected bya conduit 25 to a hopper 26 for purposes which will appear in moredetail hereinafter.

The hopper 26 is provided in communication with an opening 27 in theside of the barrel 16. An opening 28 is provided for introducing gasunder pressure into the interior of the hopper 26 and the same opening,or an additional opening 29, may be provided for evacuating the hopper26. An upper hopper, or upper chamber, 30 may, if desired, be providedfor reception therewithin of granular thermoplastic material. Said upperhopper may be opened and closed at its lower end by a valve member 31operated in any conventional, convenient manner, such as by a pressurecylinder 32 shown in FIG. 8.

Referring now to the screw 18 (FIG. 2), the zone I thereof is providedwith any conveying flights by which granular thermoplastic material fromwithin the hopper 26 is conveyed into the subsequently followingportions of the screw. The zone II is provided with flights of propershape and appropriate root diameter to effect some compression, i.e.,frictional working under pressure, of the thermoplastic material andheat generation therewithin. This at least commences the melting of thegranules. The zone III is provided with any desired flights and rootdiameter configuration to subject the thermoplastic material to thenecessary working and pressure as to complete the heating thereof andholding as hereinafter further described the gas bubbles entrainedtherein.

A pair of opposite directed discharge openings 36 and 37 extend fromeither side of the screw chamber 17 near the discharge end thereof andare preferably spaced diametrically from each other.

All aspects of the screw as above-mentioned are well known in screwdesign and may be readily provided in many forms, according to theparticular plastic being handled, by those skilled in the art to meetthe several criteria referred to above. Accordingly, it is believed thatthe screw is sufficiently described at this point and no additionaldetailing thereof is needed.

In operating this portion of the invention and creation of a foamthereby a quantity of granular, thermoplastic material will be suppliedto the upper hopper 30, either through a suitable opening (not shown)through the cover 30 thereof or by temporary removal of said cover. Thecylinder 32 is actuated to open the valve 31 and a measured quantity ofthermoplastic material, if desired the full capacity of the upper hopper30, is permitted to enter into the lower hopper. The valve 31 is thenclosed and the upper hopper may then be refilled.

Assuming the thermoplastic material to be reactive with oxygen but inertto nitrogen, vacuum from the vacuum source V will now be applied to theopening 29 so as to draw out most of the air within the hopper 26. Whensame is completed said opening will be closed as by a valve 39.

A suitable gas, such as nitrogen, under pressure is now introduced fromthe source G through the opening 28 into the interior of the hopper 26.Inasmuch as the thermoplastic granules have many spaces therebetween thegas will penetrate through said spaces and thus permeate the entiregranular mass. With such gas being under a higher pressure than the backpressure generated within the screw, the granules as they drop down ontoand into the portion of the section I of the screw continue to besurrounded by gas and said gas moves with such granules as same moveinto section II of the screw. Further with the pressure so maintainedwithin the hopper 26, as the granules commence to melt in the section IIof the screw the gas continues to permeate the material mass. Finally,when the granules complete their plastication, regardless of whether itoccurs in zone II or zone III of the screw, the gas is still distributedtherethrough in small pockets and so distributed in the same manner asit was back in the hopper 26, namely, substantially uniformly throughoutthe entire plastic mass. While the precise magnitudes of pressure willbe variable according to the requirements in an individual case, atypical illustration will be that the screw within the zone II thereofwill develop a pressure on the material of approximately 3,000 p.s.i.Thus to insure that the gas entrained in the material is not squeezedout therefrom and back to the hopper 26, the gas within the hopper 26will be maintained at a higher pressure, such as 5,000 p.s.i. This willinsure that the gas will remain fully entrained within the plasticmaterial throughout the plastication process and will continue as itstarts, namely, distributed uniformly in small separate pocketstherethrough.

The granular material within the hopper 26 is in this embodiment assumedto fall by gravity through the opening 27 onto and into the screw 18 butit will be evident that any further conventional means may be suppliedif desired for positively urging said granular material against and intosaid screw.

Thus, the plastic material appearing in the discharge openings 36 and 37will be of foam nature, namely, a foam having independent substantiallynoncommunicating cells of substantially uniform size and distributionthroughout. Further, the foam will be of a density having a high levelof uniformity and will flow into the discharge openings 36 and 37 andthence into the distribution system supplied thereby in response to thedischarge pressure of the extrusion screw, primarily the pressuregenerated and imposed on the plastic material by section III of saidscrew.

When a charge of granular material contained in the hopper 26 is fullydelivered to the extrusion screw it will, with the apparatus here shown,be desirable to withdraw said gas to the maximum extent feasible, eitherthrough its own supply passageway 28 or through the vacuum passageway29. When gas pressure is thus reduced to a low level, the valve 31 maybe withdrawn, a new charge of plastic placed in the lower hopper 26, thevalve 31 again closed, and the cycle repeated.

It will be recognized of course that in a commercial unit the upperhopper and valve 31, utilized here solely for purposes of simplicity inillustration, may be replaced by any of many presently known automaticdevices for introducing granular (including powdered) material eithercontinuously or intermittently as desired into the hopper 26 which willthen preferably be continuously pressurized by the inert gas.

The connection indicated at 25 between the thrust chamber 33 at therightward (as seen in FIG. 2) end of the plasticating screw 18 and theopening 28 for the hopper provides for the application of the samehigh-pressure gas toward the rightward end of said screw at the sametime and during all of the time that such high-pressure gas is presentwithin the hopper. The thrust chamber 33 is defined by the portion 17aof the plasticating chamber and the portion 18a of the plasticatingscrew, both of which project beyond the opening 27 in a directionopposite to the direction in which the granular material moves during aplasticating operation. The connection 25 ensures against escape of saidhigh-pressure gas toward the rearward (rightward) end of the screw and,in addition, assists the thrust bearing 22 by providing aleftwardly-directed pressure onto the screw opposing the rightwardlydirected pressure developed thereon during a plasticating operation.Thus, this conduit eliminates both the necessity for a thrust bearing atthe rearward end of the screw and for the provision of a high-pressuregas seal. The separator S of any conventional nature may be provided ifdesired in the line 25 to separate from said gas any solid particles ofthermoplastic which may be carried thereinto from either the chamber 17or from the interior of the hopper.

Turning now to the distributing and molding portion of the machine,attention is first directed to FIG. 3. A passageway 41 is provided onthe platen 4 communicating with the exit 37 from the extruder 10, thencethrough an expansion joint 42 to a T-block 43 which is backed against apressure block 44. A conduit 46 then extends from the T-block 43 througha further expansion joint 47 to another T-block 48. A conduit 49 thenextends from the T-block 48 through the expansion joint 51 (FIGS. 3 and4) to and through the check valve 52 into the injection unit 53.

The injection unit 53 is the central portion of what may be termed aninjection assembly generally identified by the numeral 54. Saidinjection assembly comprises the injection unit itself and a pluralityof valves therefor, of which one is indicated at 56, through whichplastic material from the injection assembly 53 is conducted to a moldcavity.

Referring now to injection assembly 53 in more detail, it is essentiallya small injection molding unit which may be of any of many known typesbut will here be illustrated and described sufficiently to insurecorrect understanding of the invention.

Referring to FIG. 4 there is provided a base 61 supporting a block 62within which is provided an injection chamber 63. The injection block 62may be fastened to the base block 61 by any of several convenient meanssuch as screws of which one is shown at 64. The base 61 is affixedrigidly with a layer of insulation 66 interposed therebetween onto thestationary platen 4 by any convenient means, such as screws of which oneis shown at 67. The conduit 49 is connected through the expansion joint51 and the check valve 52 to a passageway 68 which communicates throughthe passageway 69 with the interior of the injection chamber 63. Theinjection ram 71 is fastened by a conventional coupling 72 to theplunger 73 of an injection cylinder of which the cylinder portion isindicated at 74. Said cylinder 74 is mounted on an injection unit plate76 which is fixed by suitable rods of which one is shown at 77 to aposition spaced above but fixed with respect to the base plate 61. Theswitch 78 is vertically adjustable on a rod 79 in a position to beengaged by the coupling 72 as same attains a predetermined distanceabove the injection block 62. Such switch 78 is connected through aconventional control box B to valves V_(B) by which pressure fluidentering into the injection cylinder 71 is controlled.

The valve unit 56 and injection nozzle 88 are illustrated in FIG. 6. Thedischarge orifice 82 of the injection units 53 are connected by apassageway 83, partially shown in FIG. 6 and partially indicated by thedashed line connecting FIGS. 4 and 6, to the internal chamber 84 of saidvalve unit 56. When the valve plunger 86 is retracted said chamber thencommunicates to and through the injection nozzle 88 to whatever moldcavity the injection nozzle is then in communication. The valve rod 86is operated by any convenient automatic means such as a pressure fluidcylinder 90 which is operated in proper sequence with the rest of theapparatus by any conventional sequential means desired.

Each of the valve units 56a, 56b and 56c (FIG. 3) are identical to thevalve unit 56 and thence need no further description.

Additional injection assemblies are supplied by the discharge orifice 37of the extruder 15 and are indicated generally at 54a, 54b and 54c.These are all identical with the injection assembly 54 and hence need nofurther illustration or description. The unit 54a is supplied from theconduit 46 by a conduit 49a identical with the conduit 49. The units 54band 54c are supplied from the conduit 41 by a conduit 46a to conduits49b and 49c which conduits and connections including check valves areall identical to those above described in connection with conduits 46and 49 above.

Likewise the discharge orifice 36 of the foam generating unit 10 willnormally supply further, here four, additional injection assembliesthrough suitable conduits and check valves all of which preferablyconstitute a mirror image of the injection units 54, 54a, 54b and 54ctogether with the conduits and check valves above described inconnection therewith and hence need no further discussion ordescription.

The mold may be associated with the injection units in any desiredpattern. For example, as illustrated by the broken line M in FIG. 3, amold may be associated with a single valve assembly 56 or as illustratedby the broken line M₁ a single mold may be supplied for two such valveunits. A still further possibility is illustrated by the broken line M₂which indicates that a single mold with either single or multiplecavities as desired may be supplied for all four valves of the singleinjection assembly, here the injection assembly 54c. Likewise a stilllarger mold may be supplied for two or more injection assembliessimultaneously, even to the extent conceivably of supplying a singlecavity for all of the eight injection assemblies shown in FIG. 3.

Referring to FIG. 7 there is shown the locking means by which arelatively large mold may be held closed without the use of externalclamping pressure whereby the mold may be removed from the machine priorto such cooling of the molded part as would be required for opening ofthe mold.

In a high pressure injection molding operation, especially one involvinga foam formed by bubbles of pressurized gas, the mold is normally heldunder a high clamp pressure until the part has cooled sufficiently topermit opening of the mold. With a large part, this can involveconsiderable time and lessen the effectiveness of multi-mold (as turret)machines.

Thus, in the present machine the lower side of the upper or fixed moldhalf 15 is provided at the lower end of the sprue opening 91 with areversely flared portion 92 which then communicates through an openingof smaller diameter than the maximum diameter of the flared opening 92to an undercut portion 93. This latter then communicates with the runner94 leading to the mold cavity. Additional similar recesses correspondingto the opening 92 and undercut 93 may also be provided around the moldcavity and connected thereto by runners similar to runner 94. Sufficientthereof are provided in view of the opening pressure in a given mold tohold same closed upon chilling of the plastic in said recesses. Suitablecooling means, as water passages 99, are provided to effect rapidchilling of the plastic in said recesses well ahead of the cooling, orsetting, of the plastic in the mold cavity.

The mold may then be removed from the machine either manually or byautomatic means of any conventional type.

The plastic locked into the corners 96 and 97 of the opening abovedescribed will hold the mold tightly shut and effectively prevent samefrom springing open in response to the gas pressure therewithin. Thismakes possible the removal of the mold from the clamp of the machineprior to its cooling sufficiently to permit opening of the mold andhence makes more efficient the use of multi-mold machines. After themolded part has cooled the mold may then be opened. When the mold isopened, the locking plug will break at the narrow opening 95.

The lower portion thereof in the opening 92 may, if the reverse cut isnot too great and the plastic is of slightly elastomeric type as an ABSmaterial, be driven out as a cold slug by the next molding cycle or bymechanical means and handled in any conventional manner. The part in theundercut 97 can, if said undercut is not too great and the plastic is ofslightly elastomeric type as an ABS material, be driven past theundercut and out by appropriate application of knock-out pins, as theknock-out pin 98. Alternatively, with larger reverse cuts, or undercuts,or with use of a more rigid plastic material the walls defining the saidrecesses may be made radially retractable, as by utilizing an iris typestructure, a collet or other cam means in order to release therigidified plastic therefrom. Any convenient means, automatic or manual,may here also be employed to remove the rigidified plastic from withinthe locking recesses and the passageways associated therewith. While inthe present case the plastic supplied to said locking recesses is thesame plastic as supplied to the mold and is introduced thereinto at thesame time as the filling of the mold cavity, it will be evident thatthis is only illustrative and not limiting. Within the broader concept,the locking recesses may be supplied separately from the mold cavity andwith a different plastic material.

OPERATION

Although the operation has already been somewhat indicated above, itwill be reviewed in detail to insure a full understanding of both theprocess and apparatus aspects of the invention, again bearing in mindthat the apparatus expression of the invention illustrates one but notnecessarily the only manner of carrying out the process aspect thereof.

With thermoplastic material and a gas inert thereto under pressureintroduced into the foam generating device 10 as above described, a foamplastic is created and discharged from the conduits 36 and 37 in amanner already sufficiently above described. Looking for the present atonly the right hand side (as seen in FIG. 3) of the apparatus, plasticmaterial will flow through the conduit 41 to the T-head 43. As theconduit 41 heats it will expand at a rate different from that of theframe plate 4 and hence such expansion will be absorbed in the expansionjoint 42 in a manner which will be readily understood without furtherexplanation. The pressure block 44 is provided with convenientadjustment means, such as the manually adjustable screw 45, for limitingthe movement (here rightward) of the T-block 43 and thereby maintainingthe conduits 46 and 46a in proper alignment and insuring correctoperation of the expansion joint 42. The expansion joint 47 operates inthe same manner as expansion joint 42 and the pressure block 50 operatesin the same manner as the pressure block 44, both in its ownrelationship to the T-block 48 and in its relationship to the expansionjoint 47. Plastic material thus flows from the conduit 41 through theT-block 43, through both of the conduits 46 and 46a to the T-blocks 48and 48a, thence through the conduits 49, 49a, 49b and 49c to pass theseveral check valves, of which only check valve 52 is illustrated, andinto the chamber 63 of the several injection assemblies of which onlyinjection assembly 54 is fully illustrated. With the valves 56 closed,the injection chamber 63 fills and the ram 71 is pushed upwardly untilit strikes the actuator of the switch 78. When this occurs the control Bis actuated to open the valve V_(B) for supplying the pressure fluid tothe cylinder 74. The magnitude of such pressure may if desired be onlysufficient to oppose the incoming pressure from the line 49 but forsimplicity of the control system it is preferred that said pressure isof injection magnitude. Since each of the valves 56 is closed at thispoint, the downward urging of the ram 71 drives the plastic materialbelow said ram against the check valve 52 to close same and hold itclosed against the pressure of incoming plastic material. Thus, furtherfilling of the chamber 63 is prevented. However, assuming that thecorresponding rams in the other injection assemblies 54a, 54b and 54chave not yet contacted the switches therein corresponding to the switch78, the plastic material will continue to flow into each of theseassemblies until they are all caused to actuate their respectiveswitches and all are subjected to injection pressure in the manner abovedescribed in connection with the injection assembly 54. Thus, eachinjection chamber corresponding to the chamber 63 is filled with apredetermined amount of plastic material and termination of the fillingof each thereof may be made solely by the appropriate mechanical settingof the several switches corresponding to the switch 78. Further, it willbe recognized that the setting of each such switch is independent of thesetting of other switches and the modifying or resetting of any one, orplurality, of said switches will not affect or alter the amount ofplastic material going into the injection chambers associated with theother switches.

When all of the injection units are filled and subjected to injectionpressure, at least all of those which are to be utilized in any givenoperation (it being recognized that in some instances certain ones ofthese assemblies may be blocked by appropriate manually operable valves-- not shown), a suitable signal will emanate from the master controlpanel P (which may be conventional) and each of the pressure cylindersassociated with each of the valves 56 will be actuated to open saidvalves. With such valves so opened and with pressure already existing inthe injection cylinders 74, (or increased simultaneously with theopening of the valves 56) said pressure will drive the several rams 71downwardly and drive the foamed plastic within the cylinders 63 throughthe exit 82 therefrom and through the conduits above described to theinjection nozzle 88 and into the several mold cavities.

Again it will be recognized that the operation of each injectioncylinder is independent from the operation from each other injectioncylinder so that there is no necessity at all for having therein equalamounts of plastic material. Thus, each thereof will merely dischargewhatever plastic material it has in its respective injection chamber anda mold cavity, or portion thereof, associated with such nozzle will beappropriately filled.

It will be further recognized that throughout the entire path from thedischarge of the plasticating unit 10 through all of the distributionsystem, the injection assembly 54 and into the mold, a constant pressureis held on the plastic material and same is thereby controlled againstundesired expansion in response to the bubbles of pressurized gastherewithin.

It will be further recognized that in the present invention thedistribution through the general system from the discharge of theplasticating unit 10 to the injection chambers 63 is at a relatively lowpressure and hence power losses manifested as pressure drop in thepresent systems (which force distribution under high pressures) areavoided. Further, the problems of conduit strength, the sealing thereof,and other well known problems of high pressure plastic distribution areavoided. In this system the high pressure conduit exists only from thedischarge of the injection cylinders 63 to the mold and each of thesepassageways is relatively short and free of points requiring difficultsealing. Hence this system is much less expensive to build and/or tomaintain effectively than those in which the distribution is effectedunder high pressure by a single large injection ram.

It will be recognized that the machine herein described and illustratedis of extreme flexibility for the supplying of single or multiple moldsand for the supplying of molds of a wide range of sizes. Further, if themachine is operated only at partial capacity, it is necessary only tooperate whichever of the relatively small injection assemblies isrequired to fill such molds as are then in use. Still further, it isconceivable that it is possible for a user to install a machine of thistype having only for example two injection assemblies 54 but with acapacity in the stationary platen 4 for a larger number such as eightthereof and then to add further such injection assemblies subsequentlyas his business justifies or requires. It will also be observed thatalthough the apparatus of the invention was particularly designed forand is particularly adaptable to the molding of foam plastic, it is notconfined thereto but may be utilized additionally for molding ofconventional solid plastic materials, thus adding still further to itsversatility.

While the foregoing apparatus has assumed the use of an extruder as theplasticating means, it will be recognized that other known plasticatingdevices may also be used. For example, the extrusion screw 18 couldconceivably be replaced by a ram and torpedo combination wherebythermoplastic material in granular form with pressurized gas containedtherein could be supplied to the chamber 17 ahead of a plasticating ramwhich could then drive the material with entrained gas therein past atorpedo for plasticating thereof. This however, while recognized as apossibility is believed, at least at present, to be less desirable inview of the greater difficulties of plasticating by use solely of atorpedo.

While a particular embodiment of the apparatus aspects of the inventionhas been chosen to illustrate both the method and apparatus aspectsthereof, it will be recognized that the method of the invention may bepracticed by a wide variety of other types of apparatus and theapparatus of the invention may be manifested in a variety of specificembodiments. Accordingly, the specific disclosures herein will berecognized as illustrative only and not limiting.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. The method of foaming athermoplastic material comprising the steps:introducing a quantity ofsaid thermoplastic material in a granular form into a hopper above andcommunicating through an opening with a plasticating chamber containinga plasticating member consisting of an elongate member rotatable fordownstream advancement and plasticating of the thermoplastic materialand having an inlet end portion extending past said opening; maintainingsaid hopper in communication with said plasticating chamber through saidopening while otherwise closing said hopper against gas escape, andpermitting said granules to lie loosely in said hopper and in saidopening in an unsoftened state; supplying gas inert to the thermoplasticgranules from an external high pressure gas source directly into saidclosed hopper without prior heating or mechanical compression of thegranules and other than through said opening, and uniformly and rapidlypenetrating the spaces between the thermoplastic granules lying in saidhopper and said opening with said gas under pressure and wherein saidgas under pressure is provided as the foaming agent; preventing externalgas entry directly into the plasticating chamber at any point downstreamof said opening at all times, but permitting gas from said hopper toenter the plasticating chamber through said opening; introducing saidgranules, still uniformly interspaced with said high pressure gas andprior to any heating or softening of such granules, into saidplasticating chamber and engaging same by said plasticating member forsubsequent downstream movement in the plasticating chamber by theplasticating member and while continuing connection of the high pressuregas supply through said hopper and opening to said plasticating chamber;initiating and continuing melting said granules in said plasticatingchamber by heat generation therein by working and moving thethermoplastic material along the plasticating chamber by rotating ofsaid plasticating member, while maintaining said gasous pressure in saidhopper and in the communicating portion of said plasticating chamber ata pressure substantially exceeding the pressure applied to thethermoplastic material by said plasticating member during melting,wherein said gas remains uniformly distributed throughout the materialduring said melting and upon the completion of said melting theresultant plastic material encases uniformly distributed separatebubbles of said high pressure gas.
 2. The method of claim 1, includingthe further steps of:imposing a low pressure onto said melted material,in which said bubbles are uniformly distributed, and divergentlyconducting portions thereof in response to said low pressure through asystem of conduits to a plurality of injection assemblies, whilepositively preventing reverse flow of said material from an injectionassembly to other said assemblies and said plasticating chamber;accumulating a predetermined quantitiy of such plastic material in eachof said injection assemblies; and after said quantities are accumulatedin all of said plurality of injection assemblies imposing on each a highpressure higher than the distribution pressure and driving same into acommon mold.
 3. The method of claim 1, including, as part of saidreverse flow preventing, the step of isolating from the distributionsystem each injection unit when said quantity of plastic material isaccumulated therein, whereby each injection unit can thereupon operateindependently of each other injection unit.
 4. The method of claim 1including, prior to said granule to hopper introducing step, supplying aquantity of granular, thermoplastic material to an upper chamber abovesaid hopper, said granule to hopper introducing step including shiftinga valve into said hopper to drop a measured quantity of said granularmaterial from said upper chamber into said hopper, closing said valve toclose said hopper against subsequent high pressure gas leakage, andthereafter maintaining further granular thermoplastic material in saidupper chamber.
 5. The method of claim 4 including, following saidgranule to hopper introducing step, and where air may be harmful to thethermoplastic granules as during subsequent melting, ridding the hopperof atmospheric components, by connecting a vacuum source to the interiorof said hopper and drawing out at least most of the air therewithin andthereafter closing said hopper to said vacuum source prior to saidsupplying of said high pressure gas and without heating said hopper. 6.The method of claim 4 including, substantially upon full delivery of thegranular plastic material from the hopper to said plasticating member,reducing the gas pressure in said hopper to a low substantiallyatmospheric level to equalize hopper and upper chamber pressures andvalving a new charge of granular material from said upper chamber intosaid hopper therebelow for repetition of the above sequence of steps. 7.The method of claim 1 including, during said introducing and meltingsteps, continuously maintaining said opening between the bottom of saidhopper and plasticating chamber and continuously introducingthermoplastic granules individually surrounded by said high pressureinert gas into a first portion of said plasticating member at saidopening.
 8. The method of claim 1 in which said plasticating member is arotatable screw extending past said hopper opening, and said step ofintroducing granules into said plasticating chamber through said openingis carried out at least adjacent the upstream end of said screw, and inwhich said step of supplying gas under pressure to said hopper includesaxially applying high pressure gas to said upstream end of said screwand thereby offsetting upstream axial thrust forces of the screw andovercoming upstream movement of gas and granules along the screw fromthe hopper opening.
 9. The method of claim 1 in which said high gaspressure in said hopper is maintained at about 5000 p.s.i. for a saidpressure applied to the material by the plasticating member of about3000 p.s.i.
 10. The method of claim 1 including, the step of conductingthe bubble encasing melted material from said plasticating chamber to amold having a cavity and expanding of said material thereinto whileholding the mold parts closed by external means including applyingportions of said material to opposed undercut recesses the opposed moldpart faces, and cooling and thereby solidifying the bridging material insaid recesses more rapidly than the material in the mold cavity, to formlocking plugs holding the mold closed prior to completion of cooling ofthe material in said cavity to permit premature release of said externalmeans without inadvertent opening of the mold.
 11. The method of claim1, in which said plasticating member is a rotatable screw and said stepof introducing granules into said plasticating chamber is carried out atleast adjacent the upstream end of said screw, and including also thesteps of positively closing the end of said plasticating chamberadjacent the upstream end of said screw, driving said screw from theopposite end thereof and removing plasticated material from a pointadjacent the last-named end of said screw.
 12. The method of claim 11,wherein a portion of the closing of said first named end of saidplasticating chamber is carried out by introducing thereinto a gas underat least substantially the same pressure as that maintained in saidhopper.
 13. A method of foaming a thermoplastic material, comprising thesteps:providing a hopper in communication with a plasticating means butwherein said hopper is otherwise closed; opening said hopper andadmitting thereinto, without mechanical compression, a quantity ofunheated thermoplastic granules, then reclosing said hopper; introducinga high pressure gas, inert to the thermoplastic material, into theclosed hopper as the foaming agent and distributing the gas through thespaces between the granules and uniformly permeating the spaces in theentire quantity of granules lying in said hopper with said gas;permitting the granules to move by gravity from said hopper into aplasticating means and permitting expansion of said high pressure gasfrom said hopper into said plasticating means to maintain the uniformdistribution of the gas among the granules, and advancing and meltingsaid thermoplastic material by said plasticating means, with the highpressure gas still uniformly distributed through said material in smallseparate pockets as the foaming agent, to and through a dischargeportion of the plasticating means; upon substantially full delivery ofthe quantity of granular material interspersed with high pressure gas tosaid plasticating means, reducing the gas pressure in said hopper tosubstantially atmospheric pressure; thereafter introducing a newquantity of thermoplastic granules into said hopper, and repeating theabove sequence of steps by then closing the hopper at the point ofintroduction of granules and feeding further high pressure gas to thehopper so as to batch feed the plasticating member.
 14. A method offoaming a thermoplastic material, comprising the steps:providing aplasticating chamber, with a rotatable plasticating screw therein;admitting a quantity of granules of said thermoplastic material,uniformly interspersed with a high pressure gas inert thereto, intoengagement with the screw adjacent the upstream portion of said screw;continuously applying high pressure gas to the upstream end face of thescrew as said granules are admitted into engagement with it; rotatablydriving said screw from the downstream end thereof to plasticate saidmaterial with the interspersed gas still uniformly distributed therein,wherein the pressure of said gas substantially exceeds the pressureapplied to the material by the screw during plastication, andwithdrawing the resulting foamable material intermediate the ends of thescrew.
 15. The method of claim 14 including:introducing said quantity ofgranules loosely into a closed hopper above and communicating with saidplasticating chamber and screw, feeding said high pressure gas as thesole foaming agent from an external gas source directly to said hopper,and in said hopper uniformly penetrating the spaces between the granulesin the hopper with said high pressure gas; said admitting step includingdropping by gravity said granules, prior to any softening andcompression of said granules, through an opening in the bottom of saidhopper into said plasticating chamber, and permitting the high pressuregas to expand through said opening from said hopper to said plasticatingchamber, and engaging the uniformly inter-distributed granules and highpressure gas with said screw somewhat downstream of its upstream endface; said continuously applying step including positively closing theupstream end of said plasticating chamber beyond said upstream end faceof said screw and introducing gas, at least at said high pressuremaintained in hopper, to the closed upstream end of said plasticatingchamber and thereby preventing escape upstream along said screw of gasand granules from said hopper opening, while urging said screw in adownstream direction to at least partially offset reactive forces on thescrew by the thermoplastic material being plasticated thereby.
 16. Themethod of claim 15 wherein said continuously applying step includesproviding a common high pressure gas path between the hopper andupstream end of the screw and plasticating chamber through a routeexternal to said opening from said hopper to said plasticating chamber,providing between the upstream ends of said screw and plasticatingchamber a thrust space, introducing said high pressure gas into saidthrust space and preventing external gas inputs to said plasticatingchamber downstream of said hopper opening.